[2020], and written for IOCCC: The International Obfuscated C Code Contest.
This was awarded "Best of Show - abuse of libc" at the time[0]. See also the judges' remarks[1]:
This program consists of a single printf(3) statement wrapped in a while loop. You would not think that this would amount to much, but you would be very, very wrong. A clue to what is happening and how this works is encoded in the ASCII art of the program source.
How did we end up with printf - within a loop - being Turing-complete? Was it designed that way from the beginning? Were new features added over time until we got there?
Having something Turing-complete is surprisingly easy, and it hides everywhere. The repository have a small document that explains how you can use printf() as a computer : it can performs additions, logical union and negation, which is enough.
It was unintentional, but Ken Thompson being Ken Thompson, can't be 100% sure.
It was probably unintentional, yeah, I don't recall any mentions of early printf being overloaded to do stuff, nor is it clear why you would do that since you're using it in a much more convenient Turing-complete language already (C).
This was awarded "Best of Show - abuse of libc" at the time[0]. See also the judges' remarks[1]:
This program consists of a single printf(3) statement wrapped in a while loop. You would not think that this would amount to much, but you would be very, very wrong. A clue to what is happening and how this works is encoded in the ASCII art of the program source.
[0] https://www.ioccc.org/2020/index.html
[1] https://www.ioccc.org/2020/carlini/index.html
Alex: "Its primary purpose is to serve as The One True Debugger."
(It has certainly served me well.)
It was unintentional, but Ken Thompson being Ken Thompson, can't be 100% sure.
It was probably unintentional, yeah, I don't recall any mentions of early printf being overloaded to do stuff, nor is it clear why you would do that since you're using it in a much more convenient Turing-complete language already (C).
There is actually an interesting question here: was '%n' always in printf, or was it added at one point?
I took a cursory look at some old Unix source archives at TUHS: https://www.tuhs.org/cgi-bin/utree.pl
As far as I can tell from the PDP-11 assembly, Version 7 research Unix (relevant file: /usr/src/libc/stdio/doprnt.s) does not appear to implement it.
The 4.1BSD version of that file even explicitly throws an error, treating it as an invalid format specifier.
The implementation in a System III archive looks suspiciously similar to the BSD one, also throwing an error.
Only in a System V R4 archive (relevant file: svr4/ucblib/libc/port/stdio/doprnt.c) I found an implementation of "%n" that works as expected.
I guess it was added at some point to System V and through that eventually made it into POSIX?
This was an update to the earlier 4.3 BSD (1986) which still implemented printf() in VAX assembly instead, and doesn't support the %n feature.
So %n may have originally been implemented in 4.3 BSD Tahoe and made its way into SVR4 subsequently.